1. Field of the Invention
The present invention relates to a wireless communication apparatus.
Priority is claimed on Japanese Patent Application No. 2011-230675, filed Oct. 20, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
Recently, the number of mobile devices having a global positioning system (GPS) receiver has increased in the market. Various applications or services in which a user easily acquires current position information to assign the position information to a photo and arrange the photo on an electronic map or notify a friend of a current position are developing.
A positioning system using GPS satellites receives signals transmitted from a plurality of GPS satellites and calculates a position of a GPS receiver based on navigation messages of the GPS satellites included in the signals. The navigation messages include orbital information of the GPS satellites. Using this orbital information, the GPS receiver can compute which GPS satellite is currently flying over the GPS receiver.
When GPS positioning is performed for the first time, when a current position is significantly different from a position in which previous GPS positioning has been performed, or when a time has significantly elapsed after the last positioning has ended, the GPS receiver does not have an appropriate navigation message. Accordingly, because the GPS receiver does not know which GPS satellite is flying overhead, it is necessary to search for all GPS satellites in a round-robin mode. In general, a positioning start from this state is referred to as a cold start, and a significant time for positioning is required.
The GPS receiver records the navigation messages of the GPS satellites within the GPS receiver once positioning is performed. Thereby, because the GPS receiver can find which GPS satellite is flying overhead using the recorded navigation message during the next positioning, it is possible to significantly shorten the time required for positioning. However, a state in which the GPS receiver does not record appropriate navigation messages within the GPS receiver often occurs. For example, this occurs when the GPS receiver initially starts up after the GPS receiver is purchased, when the GPS receiver is moved and used overseas, or the like. At the time of this condition, several minutes may be required to complete positioning.
A method of shortening a time required for positioning by suppressing unnecessary radiation radio waves due to performing a function other than a positioning function (for example, see Japanese Unexamined Patent Application, First Publication No. 2004-061336) or a method of shortening a time required for overall positioning by appropriately setting a setting value of a time limit during start-up in a hot start or a cold start according to a start-up mode (for example, see Japanese Unexamined Patent Application, First Publication No. 2003-344523) is well known.
A signal radio wave from the GPS satellite is modulated by a code unique to each GPS satellite referred to as a coarse/acquisition (C/A) code. The GPS receiver side finds the C/A code included in a signal from the GPS satellite and demodulates the signal. Because the C/A code is iterated at 1,023 chips as one cycle at a frequency of 1.023 MHz, unique values are each allocated to 32 GPS satellites currently being operated.
If the GPS receiver does not have orbital information of a GPS satellite when moving to a position far away from a position during initial start-up or a position in which previous positioning has been performed, when positioning is resumed, or the like, it is not possible to estimate where the GPS satellite is currently flying. Accordingly, it is necessary to detect the C/A codes allocated to the 32 GPS satellites in the round-robin mode with a shift for every phase so as to find a GPS satellite from which a signal has been transmitted.
In addition, a position suitable for radio-wave reception is desirable to search for a signal transmitted from a GPS satellite, but a position in which the user actually performs positioning using the GPS receiver is not necessarily the position suitable for the radio-wave reception. Thus, a significant time is necessary for the GPS receiver to search for the GPS satellite according to a reception condition of a signal to be transmitted by the GPS satellite.
For the navigation message transmitted from the GPS satellite, 5 sub-frames form one frame and one frame is transmitted as a unit of data. Because the transmission rate of a signal to be transmitted from the GPS satellite is 50 bps, a time of 30 seconds is necessary for the GPS receiver to receive the navigation message of one frame. Further, because the GPS receiver will not carry out a calculation unless receiving navigation messages from four or more GPS satellites so as to perform three-dimensional positioning, a significant time is further required until the positioning calculation is carried out. In a method of performing imaging just after power is applied as in a mobile device carried by a user, particularly, a digital camera, and recording position information of an imaging point, a time required for this positioning is not in a permitted range.